Porous silicon produced by etching has been studied for over 30 years. Porous silicon can be oxidized to form silicon-on-insulator (SOI) structures. SOI structures provide electric insulation of active devices and are radiation hard. Photoluminesce from porous silicon was observed in 1984 (1). Recently, visible luminesces at room temperature from porous silicon has been observed (2). Stains produced in certain chemical etched (3) have been shown to consist of porous silicon. It has been shown recently that the stain films produce visible light at room temperature when radiated with ultraviolet light (5).
Interest in porous silicon has been stimulated by the observation of visible luminescence at room temperature which is consistent with the earlier measurements of photoluminescence at 4.2K from the porous silicon films produced by anodization. Stain etching of silicon in common aqueous etchants are also known to produce a film of porous silicon.
Anodic etching was carried out in aqueous HF followed by extended immersion in aqueous HF (2). The stain films produced by etching silicon in HF and NaNO.sub.2 or Cr.sub.2 O.sub.3 are similar to the films produced by anodic etching. This is expected since the etching of silicon in HF:HNO.sub.3 :H.sub.2 O solutions is recognized to follow the same oxidation-reduction reaction chemistry as anodic oxidation. Points on the surface of the silicon behave randomly as localized anodes and cathodes.
The light-emitting porous silicon film on silicon can be the basis of optoelectronic devices such as light-emitting diodes. However, neither stain etching, nor anodic etching lend themselves readily to standard solid state device processing. Stain etches and anodic etching are non-selective, they both etch all surfaces exposed to etchant. It is difficult to prepare a high resolution pattern such as a diode by etching through an exposed mask.
In order to fabricate devices, the patterns of porous silicon need to be formed on the surface of a silicon substrate and preferably by standard photoresist and lithographic techniques and materials now used to form devices on silicon.
The use of anodic etching in aqueous HF to form porous silicon is not satisfactory. Again the formation of patterns is restricted by the application of current to the complete substrate and the difficulty of selectively forming a high resolution pattern on the surface. Anodic etching is also limited to operation on certain types of doped silicon and to a narrow range of conditions. The method is cumbersome, difficult to control, particularly for n-type structures, and is not compatible with many standard optoelectronic silicon device structures.